A progressive cavity pump may be used as part of a production tubing string in an oil well. The pump consists of two parts, a stator and a rotor. The stator is connected into the production tubing that is lowered into the well casing and suspended in the well bore from the surface. The rotor is a helical element that rotates in a helical passage in the stator to drive oil through the stator. The rotor is suspended from and driven by a sucker rod string.
With this type of production string, the placement of the rotor in the stator is inexact. The torque loads on the sucker rod string vary dramatically during the pumping operation due to differences in gradients of fluids being pumped and the passage of solids through the pump. When the torque increases on the rod string, the consequent torsional deformation of the string produces a shortening of the string. Conversely, when torque on the string decreases, the string lengthens. This causes the rotor to move vertically up and down inside the stator. This detracts significantly from pump efficiency and leads to premature wear of the pump.
A complicating factor in dealing with rotor placement is the fact that the rotor not only rotates about its own axis but it also moves in a circular or eliptical path within the stator. This motion of the rotor may also cause the sucker rod string to whip violently within the production tubing, leading to premature wear of the production tubing and undesirable stresses on both the production tubing and the rod string. This effect may also cause the movement of the pump stator from side to side in the production casing, which is also undesirable.